1. Field of the invention
The present invention relates to a video signal processing apparatus for color television receivers and similar imaging devices.
2. Description of the prior art
Conventional video signal processors add a reference signal to the blanking period of the video signal, and then detect the reference signal to control feedback for high precision white balance adjustment, or detect the cathode current for automatic cut-off adjustment. One automatic white balance adjustment circuit using this method has been proposed in Japanese patent laid-open No. Sho 62-48891.
FIG. 13 is a block diagram of a conventional video signal processor. As shown in FIG. 13, this processor comprises a pseudo-pulse generator 78 for generating pseudopulses for drive and cut-off adjustment, a first adding unit 79 for adding the drive adjustment pseudo-pulse to the first line of the video signal, a second adding unit 80 for adding the cut-off adjustment pseudo-pulse to the last line of the video signal, a contrast and drive adjustment means 81 for controlling the video signal gain and adjusting the contrast and drive, a pedestal clamp video amplifier 82 for amplifying the video signal pedestal clamp and output, a current detector 83 for detecting the cathode current of the cathode ray tube, a first gate 84 that opens at the first line of the video signal, a second gate 85 that opens at the last line of the video signal, a first signal holder 86 for holding the output from the first gate, a second signal holder 87 for holding the output from the second gate, a first comparator 88 for comparing the first signal holder output with a reference voltage 90, and a second comparator 89 for comparing the second signal holder output with a reference voltage 91.
The operation of this video signal processor is described below starting with the contrast and drive adjustment operations.
The drive adjustment pseudo-pulse added to the first line of the video signal by the adding unit 79 is detected by the current detector 83 and first gate 84, and the resulting output is held by the first signal holder 86 for one vertical scanning period (1 V). The hold output is then compared with the reference voltage 90 by the first comparator 88. The first comparator 88 output is defined as an error voltage, and the gain of the contrast and drive adjustment means 81 is controlled so that this error voltage becomes zero (0).
The cut-off is adjusted as follows. The cut-off adjustment pseudo-pulse added to the last line of the video signal by the adding unit 80 is detected by the current detector 83 and second gate 85, and the resulting output is held by the second signal holder 87 for one vertical scanning period (1 V). The hold output is then compared with the reference voltage 91 by the second comparator 89. The second comparator 89 output is defined as an error voltage, and the pedestal clamp video amplifier 82 is controlled so that this error voltage becomes zero (0).
It is thus possible as described above to achieve stable contrast and cut-off control, and thus achieve a stable video signal processor, by means of feedback loop control whereby the voltage values of the drive adjustment pseudo-pulse and cut-off adjustment pseudo-pulse are maintained at a constant reference voltage.
In a conventional video signal processor as described above, the frequency characteristics and linearity deteriorate, and image chromaticity changes, according to the DC level setting because the gain and DC level are controlled and drive and cut-off adjustment are based on the video signal applied to the cathode. It is also necessary to increase the dynamic range at the video output stage in order to achieve a high amplitude, wide bandwidth signal. This requires a high voltage resistance, high frequency transistor, increasing power consumption and the circuit size, and introducing performance and reliability problems in high amplitude, wide band signal processing.